1. Field of the Invention
The present invention relates to an optical modulator module for communication systems and, more particularly, to a high-speed optical modulator module having an improved matching impedance.
2. Description of the Related Art
For a general 2.5 Gbps optical modulator module, a driving circuit and a transmission line having an internal impedance of 50.OMEGA. are typically used. A module of 25.OMEGA. characteristic impedance is sometimes used.
It is favorable in terms of production cost that the matching impedance be reduced to 25.OMEGA., but without changing the configuration of the 50.OMEGA. optical modulator module. However, if the matching impedance decreases, the inductive parasitic impedance of the lead frame and the bonding wire may cause the modulation characteristics of the optical modulator module used in the 2.5 Gbps system to deteriorate, thus resulting in a high transmission error rate. Referring to FIGS. 1 and 2, the above example is explained below.
FIG. 1 shows the planar structure of a conventional optical modulator module having an internal impedance of 50.OMEGA.. As shown in the drawing, the optical modulator module is packaged in such a manner that lead frame 10 and pad 20 are sealed with ceramic. Lead frame 10 includes an outer lead frame 12 exposed externally from the ceramic package, and an inner lead frame 14 sealed by the ceramic package. Onto pad 20 a substrate is attached, in which two chip resistors 22 and 24, DC biasing conductive plate 26, and a laser diode 28, are formed. Laser diode 28 is supported by submount 27. A high-speed signal is supplied to pin #12 of outer lead frame 12 via a coaxial cable. Inner lead frame 14 has the construction of a coplanar waveguide (CPW) comprised of signal lead S and peripheral ground leads G. Since the CPW is tens of times shorter than the wavelength at 2.5 GHz, the characteristic impedance is not sufficiently different from the coaxial cable's impedance; thus there is little variation in the modulation efficiency of the reflected wave. The two ground leads G of the CPW are in direct contact with the inner wall of the package, thereby reducing the common ground inductance.
One end of both chip resistors 22 and 24 are connected to signal lead S with multi-wire bonding; the other ends are connected to DC biasing conductive plate 26 with multi-wire bonding. The two chip resistors are configured in parallel; thus if the resistance of each chip resistor is 100.OMEGA., the parallel resistance is 50.OMEGA.. The multi-wire bonding reduces the parasitic inductance inherent in single wire bonding. The other end of DC biasing conductive plate 26 is connected to laser diode 28 with dual-wire bonding.
In these types of optical modulator modules, the inductance of the bonding wires, the matching impedance, and the frequency characteristic of the laser diode, are important factors pertaining to high-frequency modulation characteristics. In particular, the series inductance of the bonding wires considerably limits the maximum bandwidth by impeding current flow at high frequencies.
FIG. 2 is a graph illustrating the frequency characteristics curve of the conventional optical modulator module of FIG. 1, specifically depicting how the frequency characteristics of the module are affected by the modules matching impedance. Referring to FIG. 2, specifically the frequency characteristics of the intrinsic semiconductor laser diode, the limit frequency at which the normalized response gain is -3 dB is over 10 GHz. However, if a matching impedance of 50.OMEGA. is coupled, the limit frequency at which the normalized response gain is -3 dB is at 10 GHz, thus resulting in a reduced bandwidth. Furthermore, if a matching impedance of 25.OMEGA. is coupled, the limit frequency becomes 3 GHz, which drastically reduces the bandwidth. For this reason, when the resistance of the matching impedance is simply reduced from 50.OMEGA. to 25.OMEGA., it is difficult to expect reliable operation at 2.5 GHz due to the reduction of the external bandwidth and gain, and the effect of noise.